1. Field of the Invention
The present invention relates to an image capturing apparatus using an image sensor capable of capturing at least either a still image or moving image by two-dimensionally arrayed photoelectric conversion elements, and a control method therefor.
2. Description of the Related Art
A contrast detection method (called a blur detection method) and a phase-difference detection method (called a defocus detection method) have been proposed as general methods using a beam having passed through a photographing optical system in automatic focus detection/adjustment of an image capturing apparatus.
The contrast detection method is often used in a video movie camera (camcorder) for recording a moving image, and an electronic still camera. The image sensor is used as a focus detection sensor. This method pays attention to the output signal of the image sensor, especially information (contrast information) of a high-frequency component, and a position of the photographing optical system where the evaluation value maximizes is set as an in-focus position. However, the contrast detection method, also called a hill-climbing detection method, is not suitable for a high-speed focus adjustment operation because the evaluation value is obtained while slightly moving the photographing optical system, and the photographing optical system needs to be moved until it is revealed that the evaluation value was maximum.
The phase-difference detection method is generally adopted in single-lens reflex cameras using a silver halide film, and is a technique most contributed to practical use of an AF (Auto Focus) single-lens reflex camera. According to the phase-difference detection method, a beam having passed through the exit pupil of the photographing optical system is split into two, and the two beams are received by a pair of focus detection sensors. The defocus amount of the photographing optical system in the focusing direction is directly obtained by detecting the difference between signals output in accordance with the light reception amounts, i.e., the relative positional shift amount in the direction in which the beam is split. Once the focus detection sensor executes the accumulation operation, the defocus amount and direction can be attained to perform a high-speed focus adjustment operation. To split a beam having passed through the exit pupil of the photographing optical system into two, and obtain signals corresponding to the two beams, an optical path division means such as a quick return mirror or half-mirror is generally inserted in the image sensing optical path, and a focus detection optical system and AF sensor are arranged on the output side of the optical path division means. This structure makes the device bulky and expensive.
To solve this disadvantage, there is also disclosed a technique of adding a phase-difference detection function to an image sensor to eliminate a dedicated AF sensor and implement high-speed phase difference AF.
For example, in Japanese Patent Laid-Open No. 2000-156823, the pupil division function is added to some light receiving elements (pixels) of an image sensor by decentering the sensitive areas of their light receiving portions from the optical axis of an on-chip microlens. These pixels are used as focus detection pixels, and arranged between image sensing pixels at predetermined intervals to perform phase-difference focus detection. Since no image sensing pixel exists at portions where focus detection pixels are arranged, image information is generated by interpolation using information of peripheral image sensing pixels.
In Japanese Patent Laid-Open No. 2000-292686, the pupil division function is implemented by dividing each of the light receiving portions of some pixels of an image sensor into two, right and left, or upper and lower portions. These pixels are used as focus detection pixels (focus detection portions), and arranged between image sensing pixels at predetermined intervals to perform phase-difference focus detection. Even according to this technique, no image sensing pixel exists at portions where focus detection pixels are arranged, so image information is generated by interpolation using information of peripheral image sensing pixels.
In Japanese Patent Laid-Open No. 2001-305415, the pupil division function is provided by dividing each of the light receiving portions of some pixels of an image sensor into two, upper and lower portions. Outputs from the two divided light receiving portions are individually processed to execute phase-difference focus detection for an object having a luminance distribution in the vertical direction. This reference also discloses a technique of adding outputs from the two divided light receiving portions for use for an image sensing signal, and detecting the contrast between pixels adjacent to each other in the horizontal direction to perform contrast focus detection for an object having a luminance distribution in the horizontal direction.
Japanese Patent Laid-Open No. 2003-153291 discloses a technique of performing phase-difference focus detection for an object having luminance distributions in the horizontal and vertical directions. In this reference, focus detection elements whose light receiving portions are divided into right and left, or upper and lower portions are repetitively arranged every other line of an image sensor.
However, according to any of these references, luminance corresponding to each focus detection portion is too low to reliably detect the luminance distribution of an object. It is, therefore, difficult to perform accurate focus detection.